1. Field of the Invention
The present invention generally relates to a power amplifier, and more particularly, to a power amplifier with noise shaping.
2. Description of Related Art
FIG. 1 is a circuit block diagram of a conventional audio amplifier. Referring to FIG. 1, a power amplifier, in particular an audio power amplifier, usually adopts a negative feedback to obtain a stable gain. The feedback circuit in the circuit of FIG. 1 employs a voltage regulator 140 composed of an R-R circuit, wherein β is a feedback factor (for example, β≦1). As shown by FIG. 1, a differential-mode input signal pair of VIN and VIP of the integrator 110 takes low voltages as the operation voltages thereof, while the output operation voltage of the power amplifier stage 130 is high voltage. The voltage regulator 140 of FIG. 1 is for converting the high voltages of the power amplifier stage 130 into low voltages (due to β≦1) to be input to the integrator 110 so as to facilitate the circuit running smoothly.
Since an amplifier has been evolved from employing electron tubes to employing transistors, an audio power amplifier has encountered a most challenge problem, the problem of noise interference. Usually, an output power amplifier stage 130 would draw a large current from a power supply terminal VDD, but fail to effectively perform filtering on the power supply terminal VDD; therefore, when a gate signal is triggered and requires the power supply terminal VDD to feed a pure large current in, the large current itself can not satisfy the requirement because the large current contains noise and is not pure. At the point, the noise of the large current would be fed in the whole circuit through the voltage regulator 140. The above-mentioned problem can refer to “Adel S. Sedra & Kenneth C. Smith, Microelectronic Circuit, fourth edition, chapter 8, part 2”.
Although the circuit of FIG. 1 can utilize the voltage regulator 140 in the feedback circuit thereof to convert the high voltages from the power amplifier stage 130 into low voltages input to the integrator 110, but the output voltage VO (i.e., output voltage VOP and VON) of the power amplifier stage 130 contains high-frequency noise already. Thus, the DC component of the voltage can be reduced in a proportion (for example β=0.4, if the input voltage between the positive terminal and the negative terminal of the voltage regulator 140 is 10 V, then, the output voltage between the positive terminal and the negative terminal of the voltage regulator 140 would be 6 V), but the high-frequency noise thereof is not reduced therewith. In addition, the amount of β affects the total harmonic distortion (THD) of the circuit and THD further affects the strongpoint and weakness of the power amplifier stage, wherein when β is larger, we have a better THD; when β is less, we have a worse THD. In short, not only the problem of THD and noise interference can not be solved, but also the circuit connection between the high-voltage side and the low-voltage side becomes problematic and even the original input audio signal can not be kept.